The present invention relates to a circuit for controlling the contrast and brightness of a picture of a television receiver with a teletext (TTX) board, and more particularly to a circuit for commonly controlling the contrast and brightness of both TV and TTX pictures.
It is well known in this technical field that a TV receiver with a TTX board needs a picture control circuit to control the picture signals according to whether they are TV or TTX signals. The picture control is divided into the contrast and brightness control.
The conventional TV receiver with a TTX board includes two picture control circuits to separately control the contrast and brightness of TV and TTX picture signals. As shown in FIG. 1, a first variable resistor VR1 is to control the contrast of the TTX picture, second variable resistor VR2 the brightness of the TTX picture, third variable resistor VR3 the contrast of the TV picture, and fourth variable resistor VR4 the brightness of the TV picture. Hence, the changing of the picture source from TV to TTX or vice versa always requires additional picture controls. Although a remotely controlled TV receiver employs a microcomputer to process the user's key-input to control the contrast and brightness of the pictures by generating pulse width modulation (PWM) pulses integrated, this may not applied to a TV receiver with TTX.